This invention concerns the processing of digital video images and relates more especially to an improved method and means for achieving digital video effects.
It is well known to effect the transformation of a digital video image in such a manner that image information from one video source can be combined with that from another in order to achieve a variety of special effects. For example a frame containing a first image can be caused to move around within the frame of a second image by defining a desired trajectory within the second image frame. By defining parameters such as position, size, rotation and border size of the moving image further effects can be achieved. As will be understood by one skilled in the art of video editing such effects require the selection and manipulation of many different image parameters as made available for control in any given digital video effects apparatus. Effects such as turning pages, trails, textures, 3-D solid shapes, chroma keys, wraps, defocus, and shatters can all be achieved, and in a typical apparatus as many as four hundred different image parameters may be available for control by the editor. To edit such parameters for each frame of a moving image would be extremely labour intensive, and therefore in known digital effects apparatus controls are provided to enable the editor to set the values of selected parameters at critical points in the running time of a scene involving a given effect. Between the critical points at which the parameters are fixed, the values of the parameters are interpolated by the digital video effects apparatus in order to produce the desired course of the relevant effect. Such interpolation need not necessarily by linear but may proceed according to a function that has itself been defined by selection of an appropriate parameter.
This process is known as keyframe editing and virtually all currently used digital video effects apparatus are based on this system. The system does however have disadvantages in that unless the editing of a desired effect is very carefully planned in advance difficulty may arise when a sequence of events defined by consecutive keyframes is to be modified. This is because when a keyframe has been defined during the editing process the values of the parameters defined thereby have been fixed in the manner of a snapshot and thus the keyframe defines a limit of particular stage of interpolation for all parameters set thereby. If it should be desired to change the value of one parameter of a keyframe in order to vary a selected effect, for example, it my not therefore be possible to avoid undesirable changes in the course of interpolation of this parameter because the values of this parameter have been fixed by adjacent keyframes that may have been inserted only with the intention of governing the interpolation of other parameters. For this reason the editing of an existing stored effect may require laborious revision of many adjacent keyframes in order to achieve a desired charge.